The Questions of Developmental Biology

PARTE 1. Principles of development in biology
1. Developmental biology: The anatomical tradition
The Questions of Developmental Biology
According to Aristotle, the first embryologist known to history, science begins with
wonder: "It is owing to wonder that people began to philosophize, and wonder remains the
beginning of knowledge." The development of an animal from an egg has been a source of
wonder throughout history. The simple procedure of cracking open a chick egg on each
successive day of its 3-week incubation provides a remarkable experience as a thin band of cells
is seen to give rise to an entire bird. Aristotle performed this procedure and noted the formation of
the major organs. Anyone can wonder at this remarkable yet commonplace phenomenon, but
the scientist seeks to discover how development actually occurs. And rather than dissipating
wonder, new understanding increases it.
Multicellular organisms do not spring forth fully formed. Rather, they arise by a
relatively slow process of progressive change that we call development. In nearly all cases, the
development of a multicellular organism begins with a single cell the fertilized egg, or zygote,
which divides mitotically to produce all the cells of the body. The study of animal development
has traditionally been called embryology, from that stage of an organism that exists between
fertilization and birth. But development does not stop at birth, or even at adulthood. Most
organisms never stop developing. Each day we replace more than a gram of skin cells (the older
cells being sloughed off as we move), and our bone marrow sustains the development of millions
of new red blood cells every minute of our lives. In addition, some animals can regenerate
severed parts, and many species undero metamorphosis (such as the transformation of a tadpole
into a frog, or a caterpillar into a butterfly). Therefore, in recent years it has become customary to
speak of developmental biology as the discipline that studies embryonic and other
developmental processes.
Development accomplishes two major objectives: it generates cellular diversity and order
within each generation, and it ensures the continuity of life from one generation to the next. Thus,
there are two fundamental questions in developmental biology: How does the fertilized egg give
rise to the adult body, and how does that adult body produce yet another body? These two huge
questions have been subdivided into six general questions scrutinized by developmental
biologists:
The question of differentiation. A single cell, the fertilized egg, gives rise to hundreds of
different cell types muscle cells, epidermal cells, neurons, lens cells, lymphocytes, blood cells,
fat cells, and so on (Figure 1.1). This generation of cellular diversity is called differentiation.
Since each cell of the body (with very few exceptions) contains the same set of genes, we need to
understand how this same set of genetic instructions can produce different types of cells. How can
the fertilized egg generate so many different cell types?
The question of morphogenesis. Our differentiated cells are not randomly distributed. Rather,
they are organized into intricate tissues and organs. These organs are arranged in a given way: the
fingers are always at the tips of our hands, never in the middle; the eyes are always in our heads,
not in our toes or gut. This creation of ordered form is called morphogenesis. How can the cells
form such ordered structures?